Polyhalogenated aromatic compounds exhibit a very high chemical stability and are resistant to biodegradation. They are soluble in fatty materials and tend to accumulate in animal lipids, thus producing an increase of their concentration in the food chain. Several studies have clearly shown the intrinsic toxicity of these compounds and also their potential toxicity during a thermal treatment. When heated at a temperature from 300.degree. to 900.degree. C. in the presence of air, PCB produce dioxins and benzofurans, some isomers of which are still more toxic.
For these reasons, several institutions for environmental protection have promulgated strict regulations concerning the use of commercial compositions containing polyhalogenated aromatic compounds. Accordingly, transformer oils are regularly controlled due to the likelihood of their contamination by polyhalogenated aromatic compounds. In fact, PCB were widely used as dielectric fluids in transformers. The transformer oils and other fluids are classified according to their contamination level. The U.S. Environmental Protection Agency has promulgated rules and PCB-containing oils can be broken down into the following categories:
PCB-free oils : oils containing less than 50 ppm PCB;
PCB-contaminated oils : oils containing 50-500 ppm PCB;
PCB oils : oils containing more than 500 ppm PCB.
Oils containing more than 50 ppm PCB can be eliminated by burning in high temperature incinerators, but the latter must meet several and strict monitoring conditions. Therefore, the treatment cost is high. Moreover, the valuable oil is completely destroyed and lost.
Chemical methods have been suggested for the decontamination of oils containing PCB and/or other polyhalogenated aromatic compounds. However, these compounds are chemically stable and their dehalogenation requires the use of specific and very active reactants, namely alkali metals such as sodium, to be effective.
According to one method, the content of PCB in a mineral oil may be reduced by treating it with a sodium dispersion in a hydrocarbon. However, this method has several drawbacks, e.g. the dehalogenation reaction must be carried out under anhydrous conditions and the process is slow, even at high temperature.
Other dehalogenation processes consist of using alkali metal alkoxides in the presence of some solvents. But, even at high temperatures, these processes are only efficient for the dehalogenation of monohalogenated compounds.
It has been further proposed to destroy a halogenated organic compound by treating it with a reagent obtained by reacting an alkali metal or its hydroxide with a polyglycol and with oxygen, the alkali metal being used in at least a stoichiometric amount. There is formation of a complex alkali metal glycolatesuperoxide radical (U.S. Pat. Nos. 4,337,368; 4,353,793; 4,400,552; 4,460,797; European patent application No. 60089). These processes present some drawbacks, e.g. the decontamination temperature is high and the treated oils are degraded.
In an attempt to remedy these limitations, it has been suggested to treat halogenated organic compounds with a mixture of reactants comprising a polyethylene glycol or similar polyglycol, a base and an oxidizing agent or other source of free radicals (European patent application No. 118858). However, this mixture is not sufficiently active and the decontamination reaction must be carried out with the aid of micro-waves in order to reduce the reaction time and to preserve the intrinsic qualities of the treated oil.
Thus, there exists a need for an efficient process for the decomposition of polyhalogenated aromatic compounds with an effective reagent which is not hazardous and is easily stored. It is also necessary that the application of said process for the treatment of mineral oils containing polyhalogenated aromatic compounds achieve a fast and very effective decontamination without any degradation of the treated oil.